Recently, dielectric ceramics have been used widely as filter materials for communication apparatus with the spread of mobile communication. Such dielectric ceramics are required to have a high dielectric constant (.epsilon..sub.r), a low dielectric loss (tan .delta.), i.e. a high Q value that is an inverse number of the dielectric loss, and a small absolute value of the temperature coefficient at resonance frequency (TCF).
It is predicted that the frequency of communication systems will become higher and the shorter wavelength of the radio wave will be used. Therefore, when machining accuracy and conductor loss are taken into consideration, dielectrics having a low dielectric constant increasingly will be demanded. Conventional examples of a dielectric ceramic composition having a low dielectric constant include MgTiO.sub.3 --CaTiO.sub.3 based dielectric ceramic compositions, Al.sub.2 O.sub.3 based dielectric ceramic compositions or the like (disclosed in, for example, Japanese Patent Publication (Tokkai Hei) No. 6-92727). Furthermore, as the dielectric ceramic composition that has a low dielectric constant and can be sintered simultaneously with a conductor such as Ag having high conductivity at low temperature of 1100.degree. C. or less, a composition in which glass is added to Al.sub.2 O.sub.3 is well known.
However, in the MgTiO.sub.3 --CaTiO.sub.3 based dielectric ceramic composition, the loss is low and the TCF is close to zero, but the dielectric constant is as high as about 20. Furthermore, in the Al.sub.2 O.sub.3 based dielectric ceramic composition, the loss is low and the dielectric constant is as small as about 10, but the TCF has a large negative value. Similarly, in the composition in which glass is added to Al.sub.2 O.sub.3, the dielectric constant is as small as 10 or less, but the TCF has a large negative value. From such circumstances, dielectric ceramic compositions having a dielectric constant lower than that of the MgTiO.sub.3 --CaTiO.sub.3 based composition and a practical level of loss and TCF, and dielectric ceramic compositions having a dielectric constant as high as that of the composition in which glass is added to Al.sub.2 O.sub.3 and a TCF closer to zero have been demanded.